Lorelai calls Rory “Grinch” after she nags her mother into donating some of her own old clothes to the charity rummage sale that Lorelai is organising.
This is a reference to the 2000 Christmas film How the Grinch Stole Christmas (often just known as The Grinch), directed by Ron Howard and with Jim Carrey in the title role. The film was based on the children’s book of the same name by Dr. Seuss.
In the movie, the grumpy Grinch steals all the gifts from the inhabitants of Whoville in an attempt to ruin their Christmas (it doesn’t work). Lorelai is saying that Rory is “stealing” her outdated clothes for the rummage sale in order to ruin her life.
How the Grinch Stole Christmas came out in November 2000, so Lorelai and Rory could have seen it earlier in the winter. We already know that Jim Carrey is one of Sookie’s favourite actors, so perhaps she recommended it.